1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and particularly, to a LCD device capable of preventing a split between a light guide plate and a prism sheet disposed on the light guide plate and enhancing a concentration efficiency, by employing a birefringence optical sheet provided with first prism mountains having a low refractive index and second prism mountains formed between the first prism mountains and having a higher refractive index than that of the first prism mountains.
2. Background of the Invention
In general, a liquid crystal display (LCD) device is a representative flat display device for displaying an image by controlling light transmittance in correspondence with an image signal. However, the LCD device cannot emit light by itself. Thus, in order to visually display an image, a separate light source for emitting light from a rear surface of a liquid crystal module is required.
As such, to irradiate light from a rear surface of a liquid crystal module (LCM) onto a liquid crystal panel disposed at a front surface thereof, a light source (i.e., a lamp), a power source circuit for driving the light source and every component required for implementing uniform plane light are referred to as ‘backlight unit’. Such backlight units may be classified into two types, according to a method of irradiating light, namely, a direct type backlight unit ad an edge type backlight unit. Various researches have currently been conducted for direct type and edge type backlight units employing a surface light source, such as a light emitting diode (LED) or the like.
First, an edge type backlight unit is configured such that a light source is located at a side surface of a LCD module and light coming from the light source becomes plane light via a light guide plate. Such edge type backlight unit has a disadvantage, such as a difficulty in avoiding a degradation of an overall brightness. Hence, in order to obtain uniform brightness, it requires a more efficient light inducing system, namely, a system for inducing light up to a relatively remote distance from a light source. Also, an enhanced optical technology has been required to minimize light loss during transferring light up to a relatively remote distance from a light source.
FIG. 1 is a perspective view of a backlight unit according to the related art, FIG. 2a is a plot chart showing the emission distribution of light transmitted through a prism light guide plate and the emission distribution of light transmitted through a diffuser sheet, and FIG. 2b is a contour chart showing the emission distribution of light transmitted through a prism light guide plate.
As shown in FIG. 1, a backlight unit according to the related art includes a cold cathode fluorescent lamp 25 and a lamp housing 26 disposed at one side thereof, and a prism light guide plate 20 having one side surface mounted adjacent to the cold cathode fluorescent lamp 25. Also, a reflective plate 21 is disposed below the prism light guide plate 20, and a reverse prism sheet 22 and a protection sheet 23 are sequentially stacked on the prism light guide plate 20. Functions of the prism light guide plate 20 and various sheets will now be described.
One side surface of the prism light guide plate 20 is provided adjacent to the cold cathode fluorescent lamp 25, thus to emit light incident from the cold cathode fluorescent lamp 25 to its upper surface. The prism light guide plate 20 is configured such that it can be thinner as being apart from the cold cathode fluorescent lamp 25, in order to allow light incident from its side surface, at which the cold cathode fluorescent lamp 25 is located, to be uniformly emitted to its upper surface.
The reflective plate 21 is disposed below the prism light guide plate 20, so as to prevent a light leakage through the lower surface of the prism light guide plate 20 and simultaneously to reflect light toward the upper surface of the prism light guide plate 20
Accordingly, emission light transmitted through the prism light guide plate 20 is mainly distributed between 40° and 80° (approximately, 76°) with respect to the bottom surface, as shown in FIGS. 2a and 2b, and an emission light having the highest brightness has an emission angle of about 80°. However, in the related art LCD device, upon applying an external impact thereto, such as an impact test or the like, a friction between a prism pattern formed at a lower surface of the prism light guide plate 20 and the reflective plate 21, more precisely, an impact between the prism pattern and a lower cover (not shown) attached on the reflective plate 21 occurs. Accordingly, the prism pattern of the prism light guide plate 20 is split or such pattern is collapsed. Alternatively, a friction occurs between the prism light guide plate 20 formed of a rigid material and a prism pattern of the reverse prism sheet 22 formed of a ductile material, thereby causing a split or collapse of the prism pattern of the reverse prism sheet 22. Accordingly, the related art LCD device has a problem, so-called a white spot phenomenon that a certain region is brighter or darker than its surroundings.
Although not shown in detail in Figures, a typical prism sheet (not shown) may be provided at an upper surface of the prism light guide plate 20 to have prism mountains protruding toward a liquid crystal panel. Even in this case, as shown in FIG. 2c, to refract incident light at the prism sheet in a perpendicular direction, the first light L1, which is incident on a prism face by an angle of approximately 24° to 32° according to Snell's law, is required. However, for the prism light guide plate 20, most of light has an angle of 40°, the fourth light L4 which is not incident within 24° to 32° is refracted on the prism sheet to become the fifth light L5. The fifth light L5 is then departed from the prism sheet to proceed in a lateral direction, thereby to become the sixth light L6, which is called ‘side lobe phenomenon’, as shown in part A of FIG. 2c. Such phenomenon causes a drastic degradation of emission efficiency. Hence, a diffuser sheet (not shown) is further provided between the prism light guide plate 20 and the prism sheet in order to change the range of the main emission light of the prism light guide plate 20.
Upon employing the diffuser sheet for the above purpose, the main emission distribution of light through the prism light guide plate 20 is then changed from the range of 40° to 80° as shown in FIG. 2a to the range of 10° to 45°. Accordingly, the first light L1 having an angle of 24° to 32°, which is emitted via the prism light guide plate 20 and the diffuser sheet, becomes the second light L2 having an angle of 15° to 20° based upon a perpendicular line of the prism sheet within the prism sheet. The second light L2, which is departed from the prism sheet, contacts air to be perpendicularly refracted, thereby becoming the third light L3. Therefore, the perpendicular brightness of the prism sheet with respect to the upper surface of the prism light guide plate 20 increases. However, the emission light diverged from the range of 24° to 32°, emitted through the prism light guide plate 22, is still subjected to the side lobe phenomenon as shown in part A of FIG. 2c. That is, without using the main emission light of the prism light guide plate 20, the low concentration efficiency is still problematic.